Generally, when a hetero junction structure is grown by utilizing monocrystalline films having misfits, the thickness of the non-dislocation film which can be grown based on a stress effect caused by the misfits is limited to a certain critical thickness.
Therefore, there has been a large amount of difficulties in applying the electrical properties and optical and quantum effects of the hereto junction structure to the formation of devices.
Further, there exist limiting factors due to the stress effect, and therefore, conventionally when the thin film is grown, efforts have been concentrated on growing the non-dislocation film within the critical thickness.
When the stress effect is not removed, it results in a thermal instability.
Therefore, during the continuous slackening of the stress, the structural and electrical characteristics are aggravated due to the misfit dislocations which are grown near the boundary of the hetero junction structure.
Particularly, such an aggravation of the characteristics of the film shortens the life expectancy of the device, and therefore, it is recognized that the aggravation is very undesirable from the view point of the device application.
In an attempt to overcome the above described disadvantages, there have been proposed to remove the stress effect, or to artificially adjust the distribution of the misfit dislocations. Such proposals are disclosed in the following patents.
That is, several Japanese patents (J03034326, EP-420663 and J62095814) and U.S. Pat. No. 4,517,047 describe methods for removing or artificially adjusting the distribution of the misfit dislocations which are generated in the process of slackening the stress effect which is caused during the growing of the misfit hetero junction structure.
However, such methods are effective in inhibiting the formation of the misfit dislocations by adjusting the stress distribution by utilizing the superlattice, but the stress effect cannot be completely eliminated.
British Patent 2215514 describes a method for completely removing the stress effect.
In this patent, a trench process which has been used for electrically isolating the unit elements is utilized for isolating the whole surface of the substrate into a plurality of small areas so as to eliminate the stress effect. Further, an area in which the dislocation density is extremely low regionally is formed to use it as the device.
However, in this method, while it has the advantage of eliminating the stress effect, it is almost impossible to refill in a monocrystalline form the removed portion which is formed by the trench process. Therefore, there remains the problem that the whole face of the substrate cannot be utilized.